1. Field of the Invention
The present invention relates to a metal-brazing pulse burner used for brazing aluminum, copper or the like, and particularly, to an improvement of a metal-brazing pulse burner which reliably repeats an ignition and quenching in correspondence with a pulse control without quenching a flame of the burner which is intermittently ignited for controlling a temperature.
2. Description of the Prior Art
The present inventor has proposed to use a pulse burner for heating a work used for brazing a metal, especially an aluminum.
Such a pulse burner includes an air pump 50 for supplying a pressurized air to an air supply pipe 51, and a pressurized gas bomb 60 for supplying a pressurized gas to a gas supply pipe 52. Both the air supply pipe 51 and gas supply pipe 52 are provided with solenoid on-off valves 55, 56, respectively, which are opened and closed by control circuits having timers. In this pulse burner, the ignition and quenching are repeated at a burner head 59 to conduct a so-called pulse combustion, by intermittently supplying the air and gas at a cycle of 1 to 2 seconds.
The air and gas are continuously supplied and burned to heat a work. When a temperature rises and reaches a certain level, such increase in temperature can be suppressed by switching into the pulse combustion. At that time, it is possible to maintain the temperature of the work at a constant level by appropriately selecting a rate of on and off of the solenoid valve. However, after the solenoid valve is turned off, even if the valve is turned on, it is not ignited again if there is no pilot flame. For this reason, connected to a combustion gas supply pipe 63 is a pilot mixed gas supply pipe 64 to which a pilot gas supply pipe 53 having a flow rate control valve 57 and a pilot air supply pipe 54 having a flow rate control valve 58 are connected. With this arrangement, a pilot mixed gas is supplied to enable a continuous combustion of the pilot flame at the burner head 59 even if the solenoid valves for combustion air and gas are turned off, by always supplying a small amount of gas and air continuously to the mixed gas supply pipe 63 through the pilot mixed gas supply pipe 64.
However, this prior art pulse burner has a following problem. That is, the pilot mixed gas supply pipe 64 is opened at an intermediate portion of the mixed gas supply pipe 63 downstream from a connecting portion 61 between the combustion air supply pipe 51 and the gas supply pipe 52. Just after the solenoid valves 55 and 56 are turned on, air and gas are not mixed sufficiently, and only the air having a higher flow rate than that of the gas may flow first. Therefore, when a pilot flame is burning during off state of the solenoid valves, if these valves are turned on, only the air having higher flow rate may flow, and this air flow may blow off the pilot flame and the burner is quenched, which makes it difficult to stably conduct a pulse combustion.
More specifically, in the prior art burner, since a flow speed of air at the moment when the solenoid valves are turned on becomes faster than the combustion speed of gas, only the air flows and the gas combustion can not catch up with the air flow. As a result, the pilot flame may be blown off and the mixed gas may not be burned.
To solve the above problem, the present inventor tried to adjust the burner so that a timing for turning on the solenoid on-off valve 56 for gas became slightly faster than a timing for turning on the solenoid on-off valve 55 for air. With this adjustment, it was found out that quenching of the pilot flame could be suppressed all most completely, but that a large flame is generated at the moment of ignition, which is extremely dangerous.